Two types of organic gels are known: xerogels and aerogels. The former are made by simple evaporation (usually with heating) of solvent from the pore system of the gel. These gels usually are denser and have lower porosity than aerogels. Aerogels can be prepared in a manner similar to that for preparing xerogels. However, in the preparation of aerogels, the solvent is driven off by using supercritical extraction/drying, freeze drying, or similar methods.
Aerogels have a variety of useful properties. In particular, aerogels are known for their superior insulation properties. These materials have minimal environmental impact since they are air-filled. Furthermore, they are not subject to aging. Aerogels are a singular category of ultrafine (pore sizes in the nanometer range) cell size, low density, open-celled foams. Aerogels have continuous porosity and their microstructure with pore sizes below the mean free path of air is believed to be responsible for their remarkable thermal properties.
Typical aerogels are inorganic, such as silica, alumina or zirconia aerogels, prepared by the hydrolysis and condensation of the corresponding alkoxides. Silica aerogels have been developed as super insulating materials, e.g., for double pane windows. Organic aerogels would be expected to have an even lower thermal conductivity and therefore, perform better in insulating applications by providing less heat loss.
During the past decade, several organic aerogels have been described. U.S. Pat. No. 4,997,804 and U.S. Pat. No. 4,873,218 disclose polyhydroxy benzene-formaldehyde aerogels. U.S. Pat. No. 5,086,085 and U.S. Pat. No. 5,081,163 disclose melamine-formaldehyde aerogels. U.S. Pat. No. 5,484,818 discloses polyisocyanate based organic aerogels. U.S. Pat. No. 5,565,142 discloses the preparation of high porosity xerogels by chemical surface modification by the reaction of a precursor gel with R.sub.x MX.sub.y, where R is an organic group, M is selected from the group consisting of Si and Al, and X is halogen.
Aerogels, whether organic or inorganic, are typically prepared in a solvent. Evaporation of the liquid from the wet gel is very complex. When usual drying procedures are used, the gel network cracks and collapses. To overcome these problems, the pore liquid is removed by supercritical drying (i.e., the solvent is removed in its supercritical state) at pressures from about 4 MPa to about 22 MPa depending on the solvent used. Freeze-drying has also been used, but the resulting aerogels have only been obtained as powders.
There is a need for methods to produce materials with the properties of aerogels more efficiently, and at lower pressures, preferably, atmospheric pressure. The present invention discloses organic gels prepared by methods similar to xerogel preparation techniques but which have high porosity and low density properties of aerogels. The process for preparing these gels is accomplished without employing supercritical drying or freeze drying techniques.